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List of factors that might have caused the recent earth tremor in Accra

Kubabomlexo 06/25/2020

We all waked up to shocking news of an earth trmor that occur in the Greater Accra Region and some parts of the Central Region. The phenomenon which took place at about 10:40pm Wednesday, 24th June, 2020, occurred three times in space of 10 minutes. Some residents of Osu, Ablekuma, Adenta, Circle, Accra New Town, Legon, Dansoman, Kasoa, Haatso, Kwashiman, Teshie, Nungua, McCarthy Hill among other places felt it.

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Earth tremor occur when energy stored in elastically strained rocks is suddenly released. This release of energy causes intense ground shaking in the area near the source of the earthquake and sends waves of elastic energy, called seismic waves, throughout the Earth. Earthquakes can be generated by bomb blasts, volcanic eruptions, and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake prone areas, but the seismic waves generated by earthquakes are invaluable for studying the interior of the Earth. The Accra tremor might have caused by the following factors:

Stress and Strain

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Stress is a force applied over an area. One type of stress that we are all used to is a uniform stress, called pressure. A uniform stress is where the forces act equally from all directions. In the Earth the pressure due to the weight of overlying rocks is a uniform stress and is referred to as confining stress. If stress is not equal from all directions, then the stress is a differential stress. Three kinds of differential stress occur:

1. Tensional stress (or extensional stress), which stretches rock;

2. Compressional stress, which squeezes rock; and

3. Shear stress, which result in slippage and translation.

Stages of Deformation

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When a rock is subjected to increasing stress it changes its shape, size or volume. Such a change in shape, size or volume is referred to as strain. When stress is applied to rock, the rock passes through 3 successive stages of deformation:

􀁺 Elastic Deformation -- wherein the strain is

reversible.

􀁺 Ductile Deformation -- wherein the strain is

irreversible.

􀁺 Fracture -- irreversible strain wherein the

material breaks.

Evidence of Former Deformation Evidence of deformation that has occurred in the past is very evident in crustal rocks. For example, sedimentary layers and lava flows generally are deposited on a surface parallel to the Earth's surface (nearly horizontal). Thus, when we see such layers inclined instead of horizontal, evidence of an episode of deformation is present.

In order to uniquely define the orientation of a planar feature we first need to define two terms -strike and dip. For an inclined plane the strike is the compass direction of any horizontal line on the plane. The dip is the angle between a horizontal plane and the inclined plane, measured perpendicular to the direction of strike.

Fracture of Brittle Rocks

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Joints

Joints are fractures in rock that show no slippage or offset along the fracture. Joints are usually planar features, so their orientation can be described as a strike and dip. They form from as a result of extensional stress acting on brittle rock. Such stresses can be induced by cooling of rock (volume decreases as temperature decreases) or by relief of pressure as rock is eroded above thus removing weight.

Joints are zones of weakness, so their presence is critical when building anything from dams to highways. For dams, the water could leak out through the joints leading to dam failure. For highways the joints may separate and because rock falls and landslides.

Faults - Faults occur when brittle rocks fracture and there is an offset along the fracture. When the offset is small, the displacement can be easily measured, but sometimes the displacement is so large that it is difficult to measure.

The above factors might have been the reasons why we all waked up with the earth shaked in Accra.

Source: opera.com
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